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International audience; The French Navy Mine Warfare Office conducted a series of 8 calibrated underwater explosions in December 2018 in the Bay of Hyères, SE France. The charges were ranging from 80 to 680 kg TNT-equivalent weights and the generated mechanical waves have been recorded at 17 seismic stations (composed of velocimeters, accelerometers and/or Microelectomechanical system accelerometers) temporarily deployed within 10 km of the sources. The large sampling rate recordings (> 250 Hz) allow to clearly identify both seismic, hydroacoustic and acoustic waves on the shoreline. The hydroacoustic phases, mostly observed at the rocky sites, rapidly attenuate when propagating on land and seem to be conditioned by the bathymetry and the seabed sedimentary cover. The hydroacoustic onset times can be used very efficiently to infer explosion locations and origin times. The spectrum analyses and site response along the shore and inside a building (church) provide insights on the mitigation of the possible nuisance caused by the disposal of unexploded historical ordnance.

International audience; Unexploded historical ordnance (UXO) from World War II, that is discovered almost every week close to the French coast, must be destroyed quickly after discovery to ensure the safety of divers and ships. The favored destruction method is countermining, i.e., the use of a high-order detonation conducted by exploding an additional donor charge placed adjacent to the UXO. In the framework of a UXO countermining campaign conducted in the Rade d'Hyères (Mediterranean Sea, France) in December 2018, hydro-acoustic and seismic recording systems have been deployed to record the explosion-induced waves in water and the seismic signals on the shore, respectively (POSA project). In this expanded abstract, we present the main observations and focus on the impact of the shallow water environment (whose water depth is less than 50 m), and more specifically on the impact of the unconsolidated sedimentary layer, on the recorded signals induced by the detonation of charges with weights ranging from 80 to 680 kg TNT-equivalent. We also discuss the acoustic-to-seismic wave conversions.

International audience; Insight on wave regimes along the world's coastlines is important for virtually all coastal and nearshore marine activities, installations, planning and protection. Waves are pervasive and the dominant source of energy driving extreme sea levels, the transport of pollutants and sediments, erosion, and a major contributor to risks of flooding. We quantify the global spatio-temporal wave conditions along the world's coasts and evaluate the needs for coastal Earth Observation strategies, with the aim, notably, that the derived scales of change can contribute to optimisation of these strategies. A global dominant timescale of 30 days is found in coastal wave variability that is, on average, spatially correlated just over the synoptic 5 degrees' regional scale (≈ 550 km at the equator). This regional-scale dimension suggests that the timing and design of traditional field surveys and observations relevant to a vast array of coastal activities, and which may be expensive in terms of human resources, may be complemented by information gained from satellite Earth Observation that throws light on spatio-temporal scales of wave-energy change along the world's coastlines.

International audience; Explosive devices from World War II are discovered almost every week on the French coasts and they must be destroyed by the French Navy Mine Warfare Office. The consequences of the counter-mining on the marine environment are complex to evaluate. Depending on the environment geology, the explosive charges and their localization, the seismo-acoustic waves generated by the explosion may cause damage to infrastructures located on the coast, and under specific conditions small submarine landslides. The ANR ASTRID POSA project proposed to address the risk assessment issue on the Mediterranean coast. The main originality of this project lied in the approach used that combines geological measurements, acoustic data recorded within the sea coastal zone and seismic data recorded on the coast, together with numerical simulations of seismo-acoustic wave propagation. We propose here to present the results of such a methodology applied to the counter-mining campaign that occurred in the Rade of Hyères (France) in December 2018. We focus more specifically on the numerical simulations of seismo-acoustic wave propagation from the location of the device explosion to several seismometers deployed on the coast of Hyères. From topographical and sedimentary measurements performed in the Rade of Hyères, physical and geometrical characteristics of the marine seabed have been carefully selected as input data for numerical simulations. In addition, a particular attention has been paid to the measured characteristics of the device explosion (source). The numerical simulations have then been conducted using a spectral-element method. The impact of the explosive device charge and its location (on the seabed or in the water column), together with the impact of the marine environment properties, on the simulated signals have been studied. Finally, the numerical results have been compared with the real signals recorded by the seismometers. The risks of land degradation have been evaluated afterwards.

It is an illusion to think that one can observe monthly beach behaviour with monthly surveys. Current coastal observation strategies restrict understanding of beach evolution, preventing effective risk mitigation. In this article, we quantify the global spatiotemporal scales of coastal wave changes, which are the known dominant driver of beach evolution. Consequences and recommendations for beach observation strategies are proposed and discussed. A global dominant time-scale of 30 days is found driving changes in average spatially correlated just over the synoptic 5 degrees regional scale (~550 km at the equator). Current observation-practices blind us to more than 80% of unresolved beach-change variability (shorter unresolved dynamics), leading to a large unacceptable knowledge gap. This reveals common surveying-schemes --with e.g. monthly or yearly time-interval-- limit our view on the actual beach evolution, and surprisingly, even for seasonal and inter-annual evolution. The global optimal surveying time-interval --maximizing the ratio gain/effort-- is found to be semi-annual, representative of the whole regional evolution. This clearly limits the use of traditional surveying strategies and promotes a paradigm shift in observational techniques towards a large-scale use (in space and time) of space-borne Earth Observation to address this challenge.

National audience; Le projet POSA a pour but de décrire et quantifier l'énergie sismique se propageant sur les côtes lors des explosions de contre-minage menées par la Marine. En 2018, un déploiement de capteurs proches des côtes a permis d'enregistrer les champs d'ondes sur une large gamme de fréquences. Le signal lié aux explosions se caractérise par deux phases d'énergie (ondes de compression dans le milieu solide et dans l'eau). L'efficacité de la conversion eau/roche dépend fortement de la topographie et de la nature de la côte. Les stations Résif permanentes n'enregistrent que la partie basse-fréquence de ce signal.

This poster presents the POSA project (Characterization of the Propagation of Seismic Waves of Anthropic Origin, ANR ASTRID 2015-2019), which aims to describe and quantify the seismic energy propagating on the coasts during counter-mining explosions carried out by the Navy. In 2018, a deployment of sensors close to the coasts made it possible to record the wave fields over a wide range of frequencies. The signal related to the explosions is characterized by two energy phases (compressional waves in the solid medium and in the water). The efficiency of the water/rock conversion is highly dependent on the topography and nature of the coast. Permanent RESIF stations record only the low-frequency part of this signal. This poster was presented during the RESIF Scientific and Technical Meetings that took place in Biarritz in November 2019. RESIF is a national research infrastructure dedicated to the observation and understanding of the Earth's internal structure and dynamics. RESIF is based on high technology observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout the French territory. The data collected allow the study with high spatio-temporal resolution of ground deformation, surface and deep structures, local and global seismicity and natural hazards, particularly seismic, on the French territory. RESIF is integrated into the European (EPOS - European Plate Observing System) and worldwide instruments that allow to image the Earth's interior in its entirety and to study numerous natural phenomena.; Ce poster présente le projet POSA (Caractérisation de la Propagation des Ondes Sismiques d’origine Anthropique, ANR ASTRID 2015-2019), qui a pour but de décrire et quantifier l'énergie sismique se propageant sur les côtes lors des explosions de contre-minage menées par la Marine. En 2018, un déploiement de capteurs proches des côtes a permis d'enregistrer les champs d'ondes sur une large gamme de fréquences. Le signal lié aux explosions se caractérise par deux phases d'énergie (ondes de compression dans le milieu solide et dans l'eau). L'efficacité de la conversion eau/roche dépend fortement de la topographie et de la nature de la côte. Les stations RESIF permanentes n'enregistrent que la partie basse-fréquence de ce signal. Ce poster a été présenté lors des Rencontres scientifiques et techniques RESIF qui se sont déroulées à Biarritz en novembre 2019. RESIF est une infrastructure de recherche nationale dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. RESIF se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. RESIF s’intègre aux dispositifs européens (EPOS - European Plate Observing System) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels.

In natural rivers, flow discharge may fluctuate across a range of time scales – from diurnal to seasonal - but is often most pronounced during hydrographs that may encompass long-duration floods. Under these varying flows, bedforms can be created and modified by the flow without achieving any ‘equilibrium’ state. A lag between changes in flow and the morphological response of the bedforms, termed bedform hysteresis, is commonly present. Importantly for channel management and navigation, since dunes may grow larger during floods, but often experience a lagged decay in size during lowering water levels, critical water depths may be reached for inland shipping. There is also a consensus that dunes possess a more flattened shape, and lower leeside angle, than previously assumed in large rivers and that such dunes do not exhibit a region of permanent flow separation downstream of the dune. This different leeside shape thus questions traditional ideas of flow interactions with dunes, where flow separation in the steep dune lee side leads to energy loss (form drag) that increases flow resistance and energy expenditure within the flow. This paper quantifies dune hysteresis in the River Waal, Netherlands, by investigating how dune morphology changes through flood hydrographs, using high spatial- and temporal- resolution bathymetric data and robust computational analysis methods to produce probability density functions of dune morphology. This quantification aims to provide a better understanding of dune hysteresis in large rivers. The analysis examines several large data sets of river bathymetry from a 13 km reach of the River Waal, Netherlands, in a time series of bi-weekly multibeam echo sounder surveys over the last 12 years. Parameterization includes dune height, wavelength, leeside angle and leeside shape, to assess dune kinematics and hysteresis during different flood hydrographs.

International audience; The SW termination of the Gulf of Lions (GoL) is characterized by a narrowing shelf (from 80 km in the central part to ca. 20 km at the French/Spanish border), increased hydrodynamic processes and lowered sediment supply. It has been the target of a relatively large number of high-resolution seismic and acoustic investigations, as well as grab sampling, but few long cores are presently available because relatively low sedimentation rates (compared to the central GoL) did not attract the interest for paleo-environmental studies.In preparation of a map of surficial sediments in this area, a compilation of available data was undergone. In dip section, beyond the littoral prism, 4 distinct morpho-sedimentary units are at the origin of submarine landscapes. From W to E, they include: (1) between -20 and -60 m, a (2) between 60 and -75 m, a relict, 2 to 5 m-thick mud wedge, highly eroded, that can be assigned to the Younger Dryas by analogy with similar, thicker features described and well-dated further to the east; (3) between ca. -75 m and -100 m, a set of stacked thickening seaward forced-regressed Systems Tracts linked to 100 ky and 40 ky glacio-eustatic cycles. They consist of over-consolidated muds from paleo-prodeltas and lower shorefaces, and to sand bodies reworked into large dunes, drapped by a veneer of recent muds. These sands are locallycemented and form elongated reliefs at the position of paleo-shorelines (beachrocks). A characteristic of unit 3 in that part of the GoL is that, because of important reworking by hydrodynamic processes, sequences assigned to Marine Isotope Stages (MIS) 6.2, 6.4 and 7.6 are outcropping at some places, in contrast with the central GoL where they are buried underneath a 20-50 m interval of MIS 3-MIS 2 sediments. (4) between ca. -100 m and the shelf edge (that ranges between -110 and -170 m depending on the distance from canyon heads), normally consolidated offshore muds forming a wedge up to 150 m where accommodation is sufficient (infilled canyon heads).In summary, the distribution of surficial sediments in the GoL is, in general, largely controlled by the history of sea-level changes and the position of point sources. More specifically, the western termination of the GoL exemplifies the major role of hydrodynamic processes in modifying the position and water depth of boundaries between different sedimentary units and sequences.

International audience

International audience

International audience; In the last decades, sandbanks and sand dunes of various shapes and sizes have been found anddescribed on continental shelves all over the world. These structures tend to migrate and ourunderstanding of the mechanisms regulating their dynamics is limited. These sandbanks became aconcern due to the fact that they interfere with the human activities at sea. Consequently, theirpresence has economical and environmental impacts. The purpose of this study is to automaticallydetect these dunes in DTMs (Digital Terrain Models) generated from MBES data. So far, automatictechniques aiming at describing marine dune fields have always brought information such as theheight, dune spacing or the orientation at local scale but never went down to a dune scale.Currently, the dunes are still manually digitized and then form parameters are calculated. This task istedious and time-consuming. In addition, the amount of data to process being always bigger, it getsdifficult for operators to keep the pace. To address this issue, we propose a geomorphometricapproach. Attributes such as the slope or curvatures are estimated from DTMs. A combined analysisof the metrics maps enables to extract dunes from their environment. The following step is thedetermination of parameters characterizing the shape and size for each dune. This work providesknowledge about dune fields at a detailed level. Thus, the composition, spatial distribution and otheraspects of sand banks can be obtained. Further research can be conducted in order to learn moreabout the dynamics of dune fields at an individual level.

Colloque avec actes et comité de lecture. Internationale.; International audience; La prédiction du transport d'un mélange de particules de plusieurs tailles est un point sensible d'une représentation de la dynamique sédimentaire sur de nombreux faciès en rivière ou en mer. Afin de déterminer les processus critiques qui régissent la mise en mouvement de tels mélanges et de quantifier leur contributions, des essais ont été réalisés dans un petit canal à courant. Ils ont porté sur des échantillons reconstitués, formés de sables naturels lithoclastiques bien triés ou de mélanges sableux bimodaux. L'objectif est d'observer et de quantifier les différences de comportement à l'entrainement entre un sédiment unimodal et un sédiment mélangé. Les résultats obtenus sont comparés à une sélection de formulations issues de la littérature. La série de tests a permis de mettre en évidence quelques processus particuliers du début du transport sédimentaire multiclasse dans un écoulement stationnaire. La granulométrie utilisée s'étend des sables fins aux sables très grossiers. Des paramètres du mélange tels que le rapport de taille des particules, le taux de saturation des échantillons par les grains dont la taille est la plus fine ou la porosité influent sur la modulation de la contrainte de frottement critique. Les comparaisons mesures / formules ont mis en évidence la difficulté de prédire le seuil de mise en mouvement des particules d'un mélange hétérométrique. Par conséquent, l'incertitude sur la granulométrie transportée s'accroît lorsque le frottement est proche du frottement critique d'une certaine classe de taille.

Seabed of continental shelf environments is regularly covered with a multitude of bedforms formed in response to interactions between fluid dynamics and sediment. These mobile sedimentary features have been widely studied in order to prevent damages on human activities and anthropogenic structures. In coastal areas, submarine dune dynamics is mainly controlled by tidal currents and wind and wave forcings (e.g. Le Bot et al., 2004; Idier et al., 2011). However, it is generally not obvious to distinguish which forcing is predominant in dune dynamics.

Geoacoustic parameters of the seafloor are required for accurate sonar prediction and analysis of seismic reflection profiles, especially in shallow water. They are generally established by means of empirical relations. The presented work is part of CARASEDIM, an experimental project devoted to refine these geoacoustical relations in marine sediments, including coarse sands. We focus on the results of the celerimeter prototype that has been developed for that purpose. This portative device is equipped with two emitting probes and two receiving probes allowing to transmit signals between 40kHz and 400kHz. It is designed to both laboratory and in-situ measurements. We discuss about the processing techniques, the protocole of measurement and about the first results. Some laboratory results are presented in both real coarse sands and artificial glass beads. They are compared with theoretical models of sound propagation in sediments based on various assumptions (fluid, visco-elastic, porous...).

Long-studied with respect to its sedimentological features (1897), the Bay of Brest (Western Britanny, France) is a textbook example of a tide-dominated estuary. Characterised by macrotidal conditions, this estuary system is sheltered from the open sea (Iroise Sea) by a narrow strait that partitions the wave tide influences and continental/marine inputs. Sediments are supplied to the bay both by rivers (the Aulne and Elorn rivers) and by marine tidal currents. This study presents new analyses of detailed facies and morphological patterns, based on the integration of multisource data compiling seabed sampling, swath and LIDAR bathymetry, and backscatter imagery. The Main Map, at a scale of 1:90,000, contains (1) a sedimentological distribution using the ‘Code Manche’ classification, (2) a morphological map, and (3) bathymetric mapping which presents the morphology of marine and terrestrial landforms. This work may lay the foundation for a future study on sedimentary transport in a unique and confined coastal environment.

Long-studied with respect to its sedimentological features (1897), the Bay of Brest (Western Britanny, France) is a textbook example of a tide-dominated estuary. Characterised by macrotidal conditions, this estuary system is sheltered from the open sea (Iroise Sea) by a narrow strait that partitions the wave tide influences and continental/marine inputs. Sediments are supplied to the bay both by rivers (the Aulne and Elorn rivers) and by marine tidal currents. This study presents new analyses of detailed facies and morphological patterns, based on the integration of multisource data compiling seabed sampling, swath and LIDAR bathymetry, and backscatter imagery. The Main Map, at a scale of 1:90,000, contains (1) a sedimentological distribution using the ‘Code Manche’ classification, (2) a morphological map, and (3) bathymetric mapping which presents the morphology of marine and terrestrial landforms. This work may lay the foundation for a future study on sedimentary transport in a unique and confined coastal environment.

Cenozoic is marked by climatic instability and accelerating continental uplifts. Increasing sediment yield is generated, buffered and transferred by river to the deep sea. Four of the major turbidite systems in the world are located on the northern and the western margins of the Indian Ocean: the Ganges-Brahmaputra, the Indus, the Zambezi and the Tanzanian systems. This work aims to understand forcings affecting sedimentation and morphology of two main turbidite systems (Tanzanian system and Ganges-Brahmaputra system) and results have been compared with their nearest neighbors (respectively Zambezi and Indus). Based on a multiproxies approach in both area, our main results are as follows: (1) during Cenozoic, tectono-sedimentary evolution of the Tanzania margin led to the development of an atypical turbidite system wherein morphology attest of a strong structural control; (2) the Ganges-Brahmaputra system has a sedimentary activity mainly forced by sea level variations. This turbidite system records variations in many forcings impacting sedimentation from the catchment to the deep sea. Turbidite system activity and architecture revealed a strong variability mainly linked to the geodynamic and physiographic context of each studied margin. It appears necessary to integrate all forcings (affecting both the catchment and the deep sea) in the study and the understanding of sedimentary activity in a turbidite system., Le Cénozoïque est marqué par une instabilité climatique et l’accélération des surrections continentales. Ces phénomènes engendrent une augmentation de la production sédimentaire, transférée par les fleuves vers le domaine marin profond. Les façades nord et ouest de l’océan Indien voient se mettre en place quatre des plus grands systèmes sédimentaires au monde : le Gange-Brahmapoutre, l’Indus, le Zambèze et le système tanzanien. Ce travail vise à comprendre les forçages agissant sur la sédimentation et l’architecture de deux des principaux systèmes turbiditiques de l’océan Indien (système tanzanien et du Gange-Brahmapoutre), pour ensuite les comparer avec leurs plus proches voisins (respectivement le Zambèze et l’Indus). Nos principaux résultats, basés sur une approche multiproxy dans les deux zones, mettent en évidence plusieurs points : (1) l’évolution tectono-sédimentaire de la marge tanzanienne au cours du Cénozoïque a mené au développement d’un système turbiditique majeur et atypique, dont la morphologie moderne atteste de l’importance de l’activité tectonique sur sa construction ; (2) le système du Gange-Brahmapoutre enregistre une activité sédimentaire polyphasée, en lien principalement avec les variations du niveau marin. Ce système est capable d’enregistrer les variations de nombreux forçages, influençant à la fois les bassins versants et le domaine marin. Le fonctionnement sédimentaire et l’architecture des systèmes turbiditiques ont révélé une grande diversité selon le contexte géodynamique et physiographique des façades étudiées. La nécessité d’intégrer l’ensemble des forçages (agissant depuis le bassin versant jusque dans le domaine marin profond) dans l’étude de l’activité sédimentaire d’un système turbiditique apparait essentielle de nos jours.

Submarine dune dynamics is studied in the context of a macrotidal continental shelf subjected to the swell action. Off Dieppe (Eastern English Channel), the gravelly-sand seabed is moulded by dunes and sand banks, and characterized by gradients in hydrodynamics, morphologies and sediment grain-size, which are studied according to various spatial (grain, dune, gradient) and temporal scales (moment, semi-diurnal and semi-lunar tidal cycles, year, several decades). Multivaried analysis, carried out on morphological, sedimentary and hydrodynamic parameters, showed that the sediment grain-size and sediment availability are the main controlling factors of the morphology and the dynamics of the dunes. The inter-dune variability is more pronounced than the intra-dune variability, when sediment and benthic characteristics are considered. Over a 56-year period, repeated bathymetric surveys show a decrease in dune migration rates when observation periods are longer, implying a significant and frequent oscillation of dune displacements. On periods longer than one year, the dune movement concern the whole sedimentary bedform and the direction coincides with that of the residual of tide. On the semi-diurnal to the semi-lunar tidal cycle timescales, dune displacements are often confined in the crest zone. It seems that sedimentary budget is reorganized along the dune field and not at the scale of each single dune. Sediment flux calculations (bedload, non-uniform sediment) show that waves, even weak, can reverse residual sedimentary transport direction and increase the amount of transported sediments (from 10 to 100 times more for significant wave height of 1.5 m, and up to 1000 times more for annual waves). VHR seismic measurements make it possible to visualize dune internal architecture. 0.5 to 4 ° dipping discontinuities (second- order) constitute the dune master-bedding and their periodicity of formation is estimated to be inclued between 4 and 18 years. These periodicities are similar to those observed from the wavelet analysis carried out on the chronicles of tide and wave time-series recorded in the study area, and on a NAO index chronicle. Preserved second-order discontinuities are interpreted as erosive surfaces due to a temporary inversion of dune migration, caused by exceptional storm events. Their formation seems to obliterate several second-order discontinuities formed previously during weaker wave events. The medium term climatic fluctuations, which control the variability of the storm events, are presented as being the main factor influencing dune dynamics on the long term. This study of an exceptional dune field in the Eastern English Channel, according to various spatial and temporal scales, allows to put forward new interpretations as for modalities of the mobility of submarine dunes. These results can answer applied concerns, such as maritime safety, the hiding objects., La Manche orientale est une plate-forme continentale mégatidale soumise à l'action de la houle. Au large de Dieppe, les fonds marins sablo-graveleux modelés de dunes et de bancs sableux, et caractérisés par un gradient hydro - morpho - sédimentaire, sont étudiés selon plusieurs échelles spatiales (grain, dune, gradient) et temporelles emboîtées (instantané, cycles semi-diurne, et semi-lunaire, année, plusieurs décennies). Des analyses multi-variées, réalisées sur des paramètres morphologiques, sédimentaires et hydrodynamiques, ont montré que la granulométrie et le disponible sédimentaire sont les facteurs principaux de contrôle de la morphologie et de la dynamique des dunes. Une variabilité inter-dune beaucoup plus marquée que la variabilité intra-dune est mise en évidence à partir des caractéristiques granulométriques et benthiques. Sur une période de 56 ans, des levés bathymétriques répétés montrent un ralentissement des vitesses de migration des dunes lorsque la période d'observation considérée s'allonge, ce qui témoigne d'une oscillation importante et fréquente des déplacements des dunes. Sur les pas de temps supérieurs à un an, le corps dunaire dans son ensemble se déplace et la direction de migration des dunes coïncide avec celle du résiduel de marée. A l'échelle du cycle semi - diurne au cycle semi - lunaire, les déplacements sont souvent cantonnés à la zone de crête. Une réorganisation des stocks sédimentaires se fait à l'échelle du champ de dunes, et non à l'échelle de la dune. Les calculs de flux charriés en granulométrie étendue montrent qu'une houle, même faible, peut inverser le transport sédimentaire résiduel et augmenter les quantités de sédiments transportées (de 10 à 100 fois plus pour une hauteur significative de houle de 1,5 m, et jusqu'à 1000 fois plus pour des houles annuelles). Des mesures de sismique THR permettent de visualiser des discontinuités, pentées de 0,5 à 4 ° (2nd ordre), qui constituent le maître-litage des dunes et dont la périodicité de formation est estimée entre 4 et 18 ans. Ces périodicités sont similaires à celles observées à partir des analyses en ondelettes réalisées sur les chroniques de marée et de houle de la zone d'étude, et sur une chronique d'indice NAO. Les discontinuités de 2nd ordre, préservées dans l'architecture interne dunaire, sont interprétées comme des surfaces érosives dues à une inversion temporaire de migration des dunes, provoquée par des évènements de tempêtes exceptionnelles. Leur mise en place semble oblitérer plusieurs discontinuités de 2nd ordre formées précédemment pendant des épisodes de houles plus faibles. Les fluctuations climatiques à moyen terme qui contrôlent la variabilité des évènements de tempête sont présentées comme étant le facteur principal influençant la dynamique des dunes sur le long terme. Ce champ dunaire exceptionnel de la Manche orientale, permet par cette étude à échelles spatiale et temporelle emboîtées, d'avancer de nouvelles interprétations quant aux modalités de la mobilité des dunes sous-marines. Ces résultats peuvent répondre à des préoccupations appliquées telles que la sécurité maritime, l'enfouissement ou le dégagement d'objets.

In the framework of the HERMES European project, this study aims to characterize the sedimentary processes acting in the Var turbiditic system, their impact on the morphology, and their evolution through time and space. This work uses data acquired along the system, including interface cores, acoustic data, and measurement in the water column (temperature, particulate fluxes, speed and direction of the currents) thanks to mooring lines. The Var turbiditic system is under the influences of the Northern Current and of the Var river by a direct connection. Sedimentary processes are related to gravity-driven currents, oceanic currents which remobilizes sediments and hemipelagic decantation. Gravity-driven processes are dominant and are responsible of 80% of the mean annual sedimentary transport in the system, at the scale of two years (2005-2007). They are characterized by a mean speed of 100 to 600 mm.s-1 and a mean particulate flux of 10 to more than 400 g.m-2.j-1. The combination of the river discharge data and our in-situ measures allows to discriminate the mechanism responsible of their triggering: mass-wasting or river floods. Hyperpycnal turbidity currents, generated by the plunging of the turbid plume are observed for flood extended to the whole hydrographic system, with a river discharge of more than 306 m.s-1 at the river mouth. The past sedimentary processes have also been addressed, by studying the sedimentary deposits. During the last century, the canyon and the valley floor is a pathway for sediments and shows numeral erosive bedforms. Actual sedimentation is episodic and patchy and consists on coarse turbidites. Due to its elevation, the levee is a depositional area, but where only few gravity currents are able to spill over. The dominant facies of inner terraces depends on their elevation above the canyon or channel floor. On low-elevation terraces, erosional processes counterbalance depositional processes. All deposits show erosional bases. Only a few deposits on high-elevation terraces show erosional bases and they record most of the recent processes. Five major types of deposits are recorded in the deposits. They are related to (1) concentrated gravity-currents, and their evolution in (2) high-density turbidity currents, related to large slope failures, (3) hyperpycnal turbidity currents, and (4) surge-like turbidity currents, triggered by shallow retrogressive failures. A fifth kind of sedimentary deposits could be related to the decantation of a turbid plume or to an alteration by bioturbation or bottom currents. These gravity-driven currents are classified in two main groups, as a function of their impact on the morphology. Low-magnitude flows, among which surge-like turbidity currents and hyperpycnal turbidity currents related to yearly floods, constitute the main part of the daily sediment flux. These events are not powerful enough to carry their sediment load into the deep sea, and remain confined in the upper part of the turbidite system (Canyon and Upper Valley). They deposit thin, fine-grained and discontinuous sedimentary sequences, even at a metric scale, and are moreover easily eroded and hardly preserved. High-magnitude flows, among which turbidity currents related to large slope failures or to high-magnitude floods, generate significant changes in the morphology of the turbidite system. They lead to a strong erosion in the canyon and in the valley and provide thick, continuous and recognizable bodies with an extent covering the whole Var system., Le système turbiditique du Var présente la particularité d’être fréquemment parcouru par des écoulements gravitaires. Cela en fait un endroit privilégié pour étudier simultanément les facteurs déclenchant des courants de turbidité, leurs caractéristiques hydrodynamiques, leur contenu particulaire et les dépôts associés. Dans le cadre du projet Européen HERMES (6ème Programme Cadre), nous avons acquis pendant 2 ans dans le canyon du Var et dans la vallée turbiditique (1) des séries temporelles dans la colonne d’eau (30 m et 400 m au dessus du fond) sur la vitesse des courants, la température et le flux particulaire, et (2) des prélèvements répétés par carottage du sédiment de surface. Nos résultats mettent en évidence deux types de transport sédimentaire: (1) des écoulements gravitaires, et (2) des remises en suspension sur le fond par le Courant Nord. L'étude des caractéristiques hydrodynamiques des courants gravitaires permet de préciser nos connaissances sur leur mécanisme de formation. Les courants gravitaires peuvent être initiés par des déstabilisations dans le canyon ou par le plongement en mer de l'eau turbide du fleuve (courant de turbidité hyperpycnal). Ces derniers peuvent être générés lorsque le débit liquide du fleuve franchit un seuil de 306 m3.s-1 à l'embouchure, et à condition que la crue soit étendue à l'ensemble du réseau hydrographique. L’étude combinée des mesures sur les deux années et des faciès sédimentaires dans les dépôts du siècle dernier souligne la prépondérance des courants de turbidité hyperpycnaux, qui représentent 80 % des écoulements gravitaires. De plus, cette étude combinée montre que les écoulements de faible magnitude (résultant de petites instabilités ou de crues annuelles du fleuve) s'arrêtent généralement au débouché de la vallée supérieure et ne déposent du sédiment que sur une terrasse située à 70 m au dessus du fond de la vallée. Les courants gravitaires de grande magnitude (résultant de larges instabilités ou de crues majeures du fleuve) sont fortement érosifs dans la partie du système située avant le pied de pente, et déposent des séquences sédimentaires sur les terrasses, la levée, et localement dans le chenal. Le cadre chrono-stratigraphique établi pour l'étude des séquences sédimentaires permet de mieux contraindre la manière dont ces courants gravitaires sont enregistrés et préservés, à plusieurs échelles de temps.

Extensive areas of the bottom of Lake Geneva are covered with bedforms that have been referred to as pillow-hollows and quilted-cover pattern by prior observers, as well as with sediment waves and trenches. These structures are decimeter to meter scale. A large dataset of video recording and stereographic camera imagery of the lake bottom was recently acquired during a campaign with the Russian MIR submersibles. We present a classification of the different types of sediment structures with a focus on distinctive morphological characters. The variations in the observed lake-bottom structures reveal a continuous range of morphologies between the aforementioned bedforms. Although the role of the bottom-dwelling Burbot fish in at least maintaining the bottom landscape has been suggested in previous studies, the origin of the observed bedforms is unclear. On the basis of our preliminary observations, other candidate mechanisms are briefly mentioned, including bottom currents generated by internal waves. Understanding the formation and evolution of lake-bottom morphology is important since the transport pathways of lake sediment condition to a large extent the fluxes and cycle of pollutants.

The Ionian Sea is an active geodynamic area because of the convergence between theNubia and the Eurasia plate. It corresponds to the last stage of the Tethys ocean life. Therefore,the tectonics and the sedimentation are much reduced; and the low sedimentary supply enables torecord a multiplicity of sedimentary processes. Nevertheless, this area still poorly studied from asedimentary point of view, in particular on timescales on the order of hundred thousand years.This work is based on a sedimentological study of marine archives supplemented by acoustic data(bathymetry and multibeam imagery) recovered during oceanographic campaign leaded by theSHOM. The detailed sedimentary analysis of facies and sequences allows, at first, to established acalendar of the natural hazard (earthquakes, tsunamis, volcanism), their origins and theirdepositional processes into the basin over the last 330 000 years. Then, secondly, to reconstructthe evolution over the last 400 000 years of the circulation and the oxygenation of bottom waterthrough the Ionian basin and the influence of the strait of Sicily, in particular of the Malta Plateau,on the exchanges between the western and the eastern basins. The integration of thesedimentological data in a geoacoustic modelling developed by the SHOM finally allowed todetermine the impact of the sedimentary variability (special distribution, lithology, stratification)on the acoustic waves propagation for various frequency bands (300 Hz - 3000 Hz) and incidentangle (0 - 90°) and to established a mapping of the sedimentary answer of the acoustic signal., La mer Ionienne est une zone à la géodynamique active en raison de la convergence entre les plaques Nubie et Eurasie. Elle correspond aux derniers stades de vie d’un océan, la Téthys. De ce fait, la tectonique et la sédimentation y sont très réduites ; et les faibles flux sédimentaires permettent ainsi d’enregistrer une multiplicité de processus sédimentaires.Néanmoins, c’est une zone encore peu étudiée d’un point de vue sédimentaire, notamment sur les échelles de temps de l’ordre de la centaine de milliers d’années. Ce travail est basé sur une étude sédimentologique d’archives marines complétée par des données acoustiques (bathymétrie et multi-faisceau) issue de campagnes océanographiques du SHOM. L’analyse détaillée des faciès et des séquences sédimentaire a permis, dans un premier temps, d’établir un calendrier des risques naturels (séismes, tsunamis, volcanisme), leurs sources et leurs processus de dépôt dans le bassin pour les derniers 330 000 ans. Puis, dans un second temps, de retracer l’évolution sur les derniers 400 000 ans de la circulation et de l’oxygénation des eaux de fond dans le bassin ionien et l’influence du détroit de Sicile, et notamment de la plate-forme de Malte, sur les échanges entre les bassins occidental et oriental. L’intégration des données sédimentologiques dans un modèle géoacoustique développé par le SHOM a finalement permis de déterminer l’impact des variations sédimentaires (distribution spatiale, lithologie, stratification) sur la propagation des ondes acoustique pour différentes gammes de fréquences (300 Hz - 3000 Hz) et d’angle d’incidence (0 -90°) et d’établir une cartographie de la réponse sédimentaire du le signal acoustique.